The present invention relates to a pretensioner for vehicle seat belts, and in particular to a pretensioner suitable for pretensioning a belt by pulling on a buckle to which the belt is connected.
In some seat belt systems used in vehicles such as automobiles, a pretensioner is provided for preventing movement of the body of an occupant through tightening of the belt when excessive acceleration is applied on the vehicle and thereby providing greater restraint of the occupant. A pretensioner can be installed at various locations in a seat belt system. In particular, it is useful for improving occupant restraint when a buckle tongue positioned at the juncture between a shoulder section and a lap section of a belt webbing is pulled on, because tension is imparted uniformly to both belt sections. Thus, a buckle pretensioner has been proposed to pull the buckle, into which the tongue is inserted when the belt is put on.
For example, buckle pretensioners are disclosed in Japanese Patent Laid-Open Publications No. 61-241234, No. 1-119455, No. 1-164651, and No. 2-256550. A pretensioner is also disclosed by the Japanese Utility Model Laid-Open Publication No. 63-35017, although it is not designed to pull on the buckle. Those prior art pretensioners use torsion bars, compression coil springs, spiral springs, air cylinders and the like as driving devices and pull the buckle (or another element of the belt system) through a connection member by a driving force.
The driving devices of belt pretensioners provide powerful driving forces that act within very short times. Thus, to secure a certain stroke necessary for imparting pretension, belt pretensioners are of large size. It is not possible to install them on the buckle in the vehicle, and they are unattractive in appearance. Accordingly, pretensioners having torsion bars are installed horizontally below the seat, and those having compression coil springs are usually installed horizontally on the side of the seat or at some other position.
However, when the pulling direction of the buckle, which is to be pulled in a longitudinal direction, is different from the operating direction of the driving device, as in many known arrangements (except the last mentioned example), there must be provided a complicated direction-changing mechanism between the driving device and the drive element. Such direction-changing mechanism will not only lead to a complicated and large pretensioner, but may decrease the driving force for pulling, which is to be transmitted from the driving device to the buckle.
In the pretensioner of Publ. No. 63-35017, referred to above, the compression coil spring serving as the driving means is arranged coaxially with the belt-pulling direction, and the compression coil spring is installed horizontally as seen from the mounting condition of the inertia body (locking device). The mechanism with such an arrangement cannot be applied directly for the pulling of the buckle, which is to be pulled in a longitudinal direction. Even if such is possible, there must be provided locking means for preventing the belt from slackening after operation of the pretensioner, and the arrangement of such means leads to an increase in the axial or radial dimensions of the entire system.